The present invention relates to photothermographic materials and an image forming method, and in particular to photothermographic materials suitable for use in printing plate making and an image forming method by use thereof.
In the field of graphic arts and medical treatment, there have been concerns in processing of photographic film with respect to effluents produced from wet-processing of image forming materials, and recently, reduction of the processing effluent is strongly demanded in terms of environmental protection and space saving. There has been desired a photothermographic material for photographic use, capable of forming distinct black images exhibiting high sharpness, enabling efficient exposure by means of a laser imager or a laser image setter. Known as such a technique is a thermally developable photothermographic material which comprises on a support an organic silver salt, light sensitive silver halide grains, and reducing agent, as described in U.S. Pat. Nos. 3,152,904 and 3,487,075, and D. Morgan, xe2x80x9cDry Silver Photographic Materialsxe2x80x9d (Handbook of Imaging Materials, Marcel Dekker, Inc. page 48, 1991).
Such a photothermographic material contains a reducible light-insensitive silver source (such as organic silver salts), a light-sensitive silver halide and a reducing agent, which are dispersed in a binder matrix. The photothermographic materials are stable at ordinary temperature and forms silver upon heating, after exposure, at a relatively high temperature (e.g., 80xc2x0 C. to 140xc2x0 C.) through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation reduction reaction is accelerated by catalytic action of a latent image produced by exposure. Silver formed through reaction of the reducible silver salt in exposed areas provides a black image, which contrasts with non-exposes areas, leading to image formation. Such photothermographic materials meet requirements for simplified processing and environmental protection.
Such photothermographic materials have been mainly employed as photographic materials mainly for use in micrography and medical radiography, but partly for use in graphic arts. This is due to the fact that the maximum density (also denoted as Dmax) of obtained images is still low and the contrast is relatively low so that desired quality levels for graphic arts have not yet been achieved. To overcome such problems, there have been attempted incorporation of hydrazine derivatives as a contrast-increasing agent into the photothermographic material to form high contrast halftone dot images but satisfactory levels have not yet achieved. In general, when the foregoing contrast-increasing agent promotes thermal development of the halftone dot-exposed photothermographic material, halftone dots often tend to be abruptly formed so that intermediate-size and large-size dots become larger than intended their dot sizes, leading to deteriorated linearity of halftone dot images.
In the laser image setter described above, coherent light such as green laser of 500 to 600 nm and long wave laser having an emission wavelength in the near-infrared region are usually employed so that photothermographic materials used therein contain sensitizing dyes sensitive to such light are employed in the photothermographic material. After subjected to thermal processing, the sensitizing dyes remain on the halftone dot images, producing problems that dot image quality or linearity is lowered, resulting to so-called deterioration due to remaining dye stain. It was found that the use of recently developed short wave laser having an emission at 350 to 450 nm to halftone dot images on the photothermographic material resulted in superior images to those obtained by commonly known long wave laser, without causing dye stains. However, satisfactory levels were not necessarily attained.
In view of the foregoing facts, the present invention was achieved. Thus, it is an object of the invention to provide a photothermographic material exhibiting superior halftone dot quality, an enhanced maximum density and superior linearity and forming high contrast images, without causing dye stain, and an image forming method by the use thereof.
The above object of the invention can be achieved by the following constitution:
1. A photothermographic material comprising on a support an organic silver salt, silver halide grains, a reducing agent, a contrast-increasing agent and a binder, which has been prepared by using an organic solvent as a main solvent in coating, wherein the photothermographic material has a residual organic solvent content of 30 to 500 mg/m2 and exhibits a sensitivity maximum at a wavelength of 350 to 450 nm; and
2. An image forming method comprising exposing the photothermographic material described above to light using a light source having a maximum emission within the wavelength region of 350 to 450 nm.
Furthermore, preferred effects of the invention were achieved by the following embodiments.
3. The photothermographic material described in 1, wherein the silver halide grains have an average grain size of not more than 0.03 xcexcm;
4. The photothermographic material described in 1 or 2, wherein the photothermographic material comprises a compound represented by the following formula (I) to (III): 
wherein R1 through R4 are each a hydrogen atom, halogen atom, nitro group, hydroxy group, alkyl group, alkoxy group, aryl group, aryloxy group, acylamino group, carbamoyl group, sulfo group, alkylthio group or arylthio group, provided that R1 and R2, or R3 and R4 may combine with each other to form a ring, and R1 through R4 may be substituted by any substituent group; 
wherein R5 and R6 are each a hydrogen atom, alkyl group or acyl group; X is xe2x80x94COxe2x80x94 or xe2x80x94COOxe2x80x94; m, n and p are each an integer of 1 to 4, R5 and R6 may be substituted by any substituent group; 
wherein A, B and C are each a substituted or unsubstituted alkyl group, aryl group, alkoxy group, aryloxy group or heterocyclic group, provided that at least one of A, B and C is represented by the following formula (IV): 
wherein R7 and R8 are each a hydrogen atom, or a substituted or unsubstituted alkyl group, aryl group, alkoxy group or aryloxy group; and
5. The image forming method described in 2, wherein the light is incoherent light.
Photothermographic Material
The thermally developable photothermographic material relating to the invention (hereinafter, also denoted simply as photothermographic material) comprises on a support a light-sensitive silver halide layer and a light-insensitive layer, the light-sensitive layer containing a hydrophilic or hydrophobic binder, an organic silver salt, silver halide grains, a reducing agent and a contrast-increasing agent; and such ingredient compounds are dissolved or dispersed in an organic solvent or water, and preferably an organic solvent as a main solvent and coated on the support such as PET (i.e., polyethylene terephthalate) to obtain the photothermographic material. The photothermographic material preferably contains a UV absorbent.
Binder
Binders suitable for the light-sensitive layer or light-insensitive layer of the photothermographic material relating to the invention is are transparent or translucent, and generally colorless. The binders are natural polymers, synthetic resins, and polymers and copolymers, other film forming media; Examples thereof include gelatin, gum arabic, poly(vinyl alcohol), hydroxyethyl cellulose, cellulose acetate, cellulose acetatebutylate, poly(vinyl pyrrolidone), casein, starch, poly(acrylic acid), poly(methyl methacrylic acid), poly(vinyl chloride), poly(methacrylic acid), copoly(styrene-maleic acid anhydride), copoly(styrene-acrylonitrile, copoly(styrene-butadiene, poly(vinyl acetal) series [e.g., poly(vinyl formal)and poly(vinyl butyral), polyester series, polyurethane series, phenoxy resins, poly(vinylidene chloride), polyepoxide series, polycarbonate series, poly(vinyl acetate) series, cellulose esters, poly(amide) series. The binders used in the invention may be hydrophilic binders or hydrophobic binders and hydrophobic binder are preferable to minimize fogging produced after thermal development. Preferred binders are polyvinyl butyral, cellulose acetate, cellulose acetate-butylate, polyester, polycarbonate, polyacrylic acid, and polyurethane. Of these, are preferred polyvinyl butyral, cellulose acetate, cellulose acetate-butylate and polyester. As described above, the use of hydrophobic transparent binder is preferred and water-soluble or water-dispersible resin may optionally be used in combination.
Organic Silver Salt
Organic silver salts contained in the light-sensitive layer of the photothermographic material are reducible silver source, and silver salts of organic acids or organic heteroacids are preferred and silver salts of long chain fatty acid (preferably having 10 to 30 carbon atom and more preferably 15 to 25 carbon atoms) or nitrogen containing heterocyclic compounds are more preferred. Specifically, organic or inorganic complexes, ligand of which has a total stability constant to a silver ion of 4.0 to 10.0 are preferred. Exemplary preferred complex salts are described in Research Disclosure (hereinafter, also denoted as RD) 17029 and RD29963, including organic acid salts (for example, salts of gallic acid, oxalic acid, behenic acid, stearic acid, palmitic acid, lauric acid, etc.); carboxyalkylthiourea salts (for example, 1-(3-carboxypropyl)thiourea, 1-(3-caroxypropyl)-3,3-dimethylthiourea, etc.); silver complexes of polymer reaction products of aldehyde with hydroxy-substituted aromatic carboxylic acid (for example, aldehydes (formaldehyde, acetaldehyde, butylaldehyde, etc.), hydroxy-substituted acids (for example, salicylic acid, benzoic acid, 3,5-dihydroxybenzoic acid, 5,5-thiodisalicylic acid, silver salts or complexes of thiones (for example, 3-(2-carboxyethyl)-4-hydroxymethyl-4-(thiazoline-2-thione and 3-carboxymethyl-4-thiazoline-2-thione), complexes of silver with nitrogen acid selected from imidazole, pyrazole, urazole, 1,2,4-thiazole, and 1H-tetrazole, 3-amino-5-benzylthio-1,2,4-triazole and benztriazole or salts thereof; silver salts of saccharin, 5-chlorosalicylaldoxime, etc.; and silver salts of mercaptides. Of these organic silver salts, silver behenate, silver arachidate and silver stearate are specifically preferred.
The organic silver salt compound can be obtained by mixing an aqueous-soluble silver compound with a compound capable of forming a complex. Normal precipitation, reverse precipitation, double jet precipitation and controlled double jet precipitation described in JP-A 9-127643 are preferably employed (hereinafter, the term, JP-A refers to unexamined and published Japanese Patent Application). For example, to an organic acid is added an alkali metal hydroxide (e.g., sodium hydroxide, potassium hydroxide, etc.) to form an alkali metal salt soap of the organic acid (e.g., sodium behenate, sodium arachidate, etc.), thereafter, the soap and silver nitrate are mixed by the controlled double jet method to form organic silver salt crystals. In this case, silver halide grains may be concurrently present.
Silver Halide Grain
Silver halide grains contained in the light-sensitive layer of the photothermographic material functions as a light sensor. In order to minimize cloudiness after image formation and to obtain excellent image quality, the less the average grain size, the more preferred, and the average grain size is preferably not more than 0.03 xcexcm, and more preferably between 0.01 and 0.03 xcexcm. The silver halide grains are preferably prepared simultaneously in the preparation of organic silver salts. It is also preferred that silver halide grains are prepared together with organic silver salt, forming silver halide grains fixed on organic silver salt grains and resulting in minute grains, so-called in situ silver. Electron-micrographs of at least 100 silver halide grains are taken at a factor of 50000 to determine the average grain size. Thus, the longest edge length and the shortest edge length of the grain are determined for 100 grains and the summation thereof divided by 200 is defined as the average grain size in the invention.
The average grain size as described herein is defined as an average edge length of silver halide grains, in cases where they are so-called regular crystals in the form of cube or octahedron. Furthermore, in cases where grains are not regular crystals, for example, spherical, cylindrical, and tabular grains, the grain size refers to the diameter of a sphere having the same volume as the silver grain. Furthermore, silver halide grains are preferably monodisperse grains. The monodisperse grains as described herein refer to grains having a monodispersibility (i.e., coefficient of variation of grain size distribution, as defined below) of not more than 40%; more preferably not more than 30%, still more preferably not more than 20%, and most preferably 0.1 to 20%:
Monodispersibility=(standard deviation of grain size)/(average grain size)xc3x97100(%)
Silver halide grains used in the invention preferably exhibit an average grain size of not more than 0.1 xcexcm, preferably not more than 0.03 mm, and more preferably 0.01 to 0.03 xcexcm, and monodisperse grains are still more preferred. The use of silver halide grains falling within such a grain size range leads to enhanced image graininess.
The silver halide grain shape is not specifically limited, but a high ratio accounted for by a Miller index [100] plane is preferred. This ratio is preferably at least 50%; is more preferably at least 70%, and is most preferably at least 80%. The ratio accounted for by the Miller index [100] face can be obtained based on T. Tani, J. Imaging Sci., 29, 165 (1985) in which adsorption dependency of a [111] face or a [100] face is utilized.
Furthermore, another preferred silver halide shape is a tabular grain. The tabular grain as described herein is a grain having an aspect ratio represented by r/h of at least 3, wherein r represents a grain diameter in xcexcm defined as the square root of the projection area, and h represents thickness in xcexcm in the vertical direction. Of these, the aspect ratio is preferably between 3 and 50. The grain diameter is preferably not more than 0.1 xcexcm, and is more preferably between 0.01 and 0.08 xcexcm. These are described in U.S. Pat. Nos. 5,264,337, 5,314,789, 5,320,958, and others. In the present invention, when these tabular grains are used, image sharpness is further improved. The composition of silver halide may be any of silver chloride, silver chlorobromide, silver iodochlorobromide, silver bromide, silver iodobromide, or silver iodide.
Silver halide emulsions used in the invention can be prepared according to the methods described in P. Glafkides, Chimie Physique Photographique (published by Paul Montel Corp., 19679; G. F. Duffin, Photographic Emulsion Chemistry (published by Focal Press, 1966); V. L. Zelikman et al., Making and Coating of Photographic Emulsion (published by Focal Press, 1964).
Silver halide preferably occludes ions of metals belonging to Groups 6 to 11 of the Periodic Table. Preferred as the metals are W; Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt and Au. These metals may be introduced into silver halide in the form of a complex.
Silver halide grain emulsions used in the invention may be desalted after the grain formation, using the methods known in the art, such as the noodle washing method and flocculation process.
The photosensitive silver halide grains used in the invention is preferably subjected to a chemical sensitization. As preferable chemical sensitizations, commonly known chemical sensitizations in this art such as a sulfur sensitization, a selenium sensitization and a tellurium sensitization are usable. Furthermore, a noble metal sensitization using gold, platinum, palladium and iridium compounds and a reduction sensitization are available.
In order to minimize haze (or cloudiness) of the recording material, the total silver coverage including silver halide grains and organic silver salts is preferably 0.3 to 2.2 g/m2, and more preferably 0.5 to 1.5 g/m2. Such a silver coverage forms a relatively high contrast image. The silver halide amount is preferably not more than 50% by weight, and more preferably not more than 25% by weight, and still more preferably 0.1 to 15% by weight, based on the total silver amount.
The silver halide grains used in the invention preferably exhibit the maximum absorption (so-called absorption maximum) at 350 to 450 nm, which may spectrally be sensitized with sensitizing dyes.
Further, the photothermographic material according to the invention exhibits a sensitivity maximum at a wavelength of 350 to 450 nm. The sensitivity maximum can be determined by subjecting a photothermographic material to absorption spectroscopy using an integrating sphere comprised of KBr. The sensitivity maximum at a wavelength of 350 to 450 nm refers to the absorption maximum being within the range of 350 to 450 nm.
Reducing Agent
Reducing agents are incorporated into the photothermographic material of the present invention. Examples of suitable reducing agents are described in U.S. Pat. Nos. 3,770,448, 3,773,512, and 3,593,863, and Research Disclosure Items 17029 and 29963, and include the following: aminohydroxycycloalkenone compounds (for example, 2-hydroxypiperidino-2-cyclohexane); esters of amino reductones as the precursor of reducing agents (for example, piperidinohexose reducton monoacetate); N-hydroxyurea derivatives (for example, N-p-methylphenyl-N-hydroxyurea); hydrazones of aldehydes or ketones (for example, anthracenealdehyde phenylhydrazone; phosphamidophenols; phosphamidoanilines; polyhydroxybenzenes (for example, hydroquinone, t-butylhydroquinone, isopropylhydroquinone, and (2,5-dihydroxy-phenyl)methylsulfone); sulfydroxamic acids (for example, benzenesulfhydroxamic acid); sulfonamidoanilines (for example, 4-(N-methanesulfonamide)aniline); 2-tetrazolylthiohydroquinones (for example, 2-methyl-5-(1-phenyl-5-tetrazolylthio)hydroquinone); tetrahydroquionoxalines (for example, 1,2,3,4-tetrahydroquinoxaline); amidoxines; azines (for example, combinations of aliphatic carboxylic acid arylhydrazides with ascorbic acid); combinations of polyhydroxybenzenes and hydroxylamines, reductones and/or hydrazine; hydroxamic acids; combinations of azines with sulfonamidophenols; xcex1-cyanophenylacetic acid derivatives; combinations of bis-xcex2-naphthol with 1,3-dihydroxybenzene derivatives; 5-pyrazolones, sulfonamidophenol reducing agents, 2-phenylindane-1,3-dione, etc.; chroman; 1,4-dihydropyridines (for example, 2,6-dimethoxy-3,5-dicarboethoxy-1,4-dihydropyridine); bisphenols (for example, bis(2-hydroxy-3-t-butyl-5-methylphenyl)methane, bis(6-hydroxy-m-tri)mesitol, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 4,5-ethylidene-bis(2-t-butyl-6-methyl)phenol, UV-sensitive ascorbic acid derivatives and 3-pyrazolidones. Of these, particularly preferred reducing agents are hindered phenols.
As preferred hindered phenols, listed are compounds represented by the general formula (A) described below: 
wherein R represents a hydrogen atom or an alkyl group having from 1 to 10 carbon atoms (for example, isopropyl, xe2x80x94C4H9, 2,4,4-trimethylpentyl), and Rxe2x80x2 and Rxe2x80x3 each represents an alkyl group having from 1 to 5 carbon atoms (for example, methyl, ethyl, t-butyl).
Exemplary examples of the compounds represented by the formula (A) are shown below. 
The used amount of reducing agents represented by the above-mentioned general formula (A) is preferably from 1xc3x9710xe2x88x922 to 10 moles, and is more preferably from 1xc3x9710xe2x88x922 to 1.5 moles per mole of silver.
Contrast-increasing Agent
The contrast increasing agent contained in the light-sensitive layer of the photothermographic material is preferably hydrazine compounds. Exemplary hydrazine compounds usable in the invention include those described in Research Disclosure Item 23515 (November, 1983, page 346) and references cited therein; U.S. Pat. Nos. 4,080,207, 4,269,929, 4,276,364, 4,278,748, 4,385,108, 4,459,347, 4,478,928, 4,560,638,, 4,686,167, 1912,016, 4,988,604, 4,994,365, 5,041,355, and 5,104,769; British Patent No. 2,011,391B, European Patent Nos. 217,310, 301,799 and 356,898; JP-A Nos. 60-179434, 61-170733, 61-270744, 62-178246, 62-270948, 63-29751, 63-32538, 63-194947, 63-121838, 63-129337, 63-223744, 63-234244, 63-234245, 63-234246, 63-294552, 63-306438, 64-10233, 1-90439, 1-100530, 1-105941, 1-105943, 1-276128, 1-280747, 1-283548, 1-28283549, 1-285940, 2-2541, 2-77057, 2-139538, 2-196234, 2-196235, 2-198440, 2-198441, 2-198442, 2-220042, 2-221953, 2-221954, 2-285342, 2-285343, 2-302750, 2-304550, 3-37642, 3-54549, 3-125134, 3-184039, 3-240036, 3-240037, 3-259240, 3-280038, 3-282536, 4-51143, 4-56842, 4-84134, 2-230233, 4-96053, 4-216544, 5-45761, 5-45762, 5-45763, 5-45764, 5-45765, 6-289524, and 9-160164.
Examples of the contrast-increasing agent further include compounds represented by (chemical formula 1) described in JP-B 6-77138, including compounds at page 3 to 4 (hereinafter, the term, JP-B refers to published Japanese Patent); compounds represented by formula (1), described in JP-B No. 6-93082, including compound No. 1 through 38 described at page 8 to 18; compounds represented by formulas (4), (5) and (6), described in JP-A No. 6-23049, including compounds 4-1 through 4-10 described at page 25 to 26, and compounds 5-1 through 5-42 described at page 28 to 36, and compounds 6-1 through 6-7 described at page 39 and 40; compounds represented by formula (1) and (2), described in JP-A No. 6-289520, including compounds 1-1) through 1-17) and 2-1) described at page 5 to 7; compounds represented by (chemical formula 2) and (chemical formula 3), described in JP-A 6-313936, including compounds described at page 6 to 19; compounds represented by (chemical formula 1) described in 6-313951, including compounds described at page 3 to 5; compounds represented by formula (I) described in JP-A No. 7-5610, including compounds I-1 through I-38; compounds represented by formula (II) described in JP-A 7-77783, including compounds described at page 10 to 27; and compounds represented by formula (H) and (Ha) described in JP-A 7-104426, including compounds H-1 through H-44 described at page 8 to 15.
Preferred contrast-increasing agents used in the invention include those described in JP-A No. 11-316437 at page 33 to 53, and more preferred compounds are those described in JP-A 12-298327 at page 17 to 25, represented by the following formulas: 
UV Absorbent
Next, the UV absorbent represented by formula (I), (II) or (III), contained in the photothermographic material relating to the invention will be described.
In formula (I), R1 through R4 each represent a hydrogen atom, halogen atom, nitro group, hydroxy group, alkyl group, alkoxy group, aryl group, aryloxy group, acylamino group, carbamoyl group, sulfo group, alkylthio group or arylthio group, provided that R1 and R2, or R3 and R4 may combine with each other to form a ring. In formula (II), R5 and R6 each represent a hydrogen atom, alkyl or acyl group; X represents xe2x80x94Cxe2x95x90 or xe2x80x94COOxe2x80x94; m, n and p are each an integer of 1 to 4. These substituent groups represented in formula (I) or (II) may be further substituted by any substituent group. 2-(2xe2x80x2-hydroxyphenyl)benzotriazole type UV absorbents used in the invention are liquid at ordinary temperature. Such liquids are exemplarily described in JP-B Nos. 55-36984 and 55-12587 and JP-A No. 214152. The atoms or groups represented by R1 through R4 in formula (I) are detailed in JP-A Nos. 58-221844, 59-46646, 59-109055; JP-B Nos. 36-10466, 42-26187, 48-5496, and 48-41572; U.S. Pat. Nos. 3,754,919 and 4,220,711. The groups represented by R5 and R6 in benzophenone type UV absorbents, represented by formula (II) are detailed in JP-B No. 48-30493 (or U.S. Pat. No. 3,698,907) and JP-B No. 48-31255.
In formula (III), A, B and C independently represent a substituted or unsubstituted alkyl group (preferably having 1 to 20 carbon atoms), aryl group, alkoxy group, aryloxy group or heterocyclic group (e.g., pyridyl). Examples of a substituent group include hydroxy, a halogen atom (e.g., fluorine, chlorine, bromine), alkyl group having 1 to 12 carbon atoms (e.g., methyl ethyl, butyl, trifluoromethyl, hydroxyoctyl, epoxymethyl), alkoxy group having 1 to 18 carbon atoms (e.g., methoxy, ethoxy, butoxy, cyclohexyloxy, benzoyloxy), aryloxy group having 6 to 18 carbon atoms (e.g., phenoxy, m-methylphenoxy), alkoxycarbonyl group (e.g., ethoxycarbonyl, 2-methoxyethoxycarbonyl), aryloxycarbonyl group (e.g., phenoxycarbonyl, p-methylphenoxycarbonyl), alkylthio group having 1 to 18 carbon atoms (e.g., methylthio, butylthio) and carbamoyl group (e.g., methylcarbamoyl, butylcarbamoyl). Of the groups represented by A, B and C, the group, other than the group represented by formula (IV), is preferably a substituted or unsubstituted aryl or alkoxy group.
R7 and R8 in formula (IV) independently represent a halogen atom (e.g., fluorine, chlorine, bromine), alkyl group having 1 to 18 carbon atoms (e.g., methyl, trifluoromethyl, cyclohexyl, glycidyl), substituted or unsubstituted aryl group having 6 to 18 carbon atoms (e.g., phenyl, tolyl), substituted or unsubstituted alkoxy group (e.g., methoxym butoxy, 2-butoxyethoxy, 3-butoxy-2-hydoxypropyloxy), and substituted or unsubstituted aryloxy group having 6 to 18 carbon atoms (e.g., phenoxy, p-methylphenoxy). R7 and R8 are preferably an alkoxy group having 1 to 20 carbon atoms, in which a substituent group is substituted preferably at the para-position to the carbon atom attached to the triazine ring.
The compound represented by formula (III) can be synthesized in accordance with the method described in JP-A No. 46-3335 or European Patent No. 520938A1. Examples of UV absorbents usable in the invention are shown below but are by no means limited to these examples.

The UV absorbent is preferably contained in the light-insensitive layer, and more preferably in the layer provided on a light-sensitive layer provided farthest from the support. The UV absorbent by formula (I), (II) or (III) may be used alone or in combination with other UV absorbent(s) having a different chemical structure, but at least two, and more preferably at least three selected from the foregoing UV absorbents of formula (I), (II) and (III) are preferably used in combination, at least one of which is still more preferably liquid. In cases where the UV absorbent is contained together with a hydrophilic or hydrophobic binder, the binder is contained preferably in an amount of 5 to 100%, and more preferably 5 to 50% by weight, based on the UV absorbent. The UV absorbent is coated preferably in such an amount that the UV absorbent exhibits an absorbance at 360 nm of at least 0.6, more preferably at least 1.0, and still more preferably at least 1.5. The UV absorbent is dispersed in a binder, preferably together with a high boiling solvent such as waxes.
Decolorizing Agent for UV Absorbent
In a photothermographic material containing a UV absorbent, in cases when at least a part of the UV absorbent remains in the photothermographic material, without being decomposed after subjected to thermal development, the residual UV absorbent often lowers efficiency of printing on a pre-sensitized plate (PD plate) by using UV rays. It is therefore preferred to incorporate the following decolorizing agent effective for decolorizing the UV absorbent in the UV absorbent-containing layer or a layer adjacent thereto.
Examples of the decolorizing agent for the UV absorbent include an adduct of bisphenol and alkylene oxide, methyloamide or bisamide having a melting point of not lower than 110xc2x0 C., long chain 1,2-glycol, an aduct of terephthalic acid and alkylene oxide, solid alcohols such as stearyl alcohol described in JP-B No. 50-17865, polyethylene glycol and 1,8-octanediol, polyethers or polyethylene glycol derivatives such as polyethylene oxide, sorbitan monostearate and oxyethylene-alkylamine, as described in JP-B 50-17876 and 50-17868, acetoamides described in JP-B No. 51-19991, stearoamides, phthalonitrile, m-nitroaniline and xcex2-naphthylamine, guanidine derivatives described in JP-B No. 51-29024, and amines or quaternary ammonium salts such as hexadecylamine, tribenzylamine, 2-aminobemzoxazole and hexadecyltrimethylammonium chloride. The foregoing decolorizing agents for UV absorbents are contained preferably in an amount of 0.05 to 8 g/m2.
Other Components in Photothermographic Material
A light-insensitive layer may be provided on the outermost side of the light-sensitive layer to protect the light-sensitive layer or prevent abrasion marks from occurring. Binders used in the light-insensitive layer may be the same as or different from those used in the light-sensitive layer.
To accelerate thermal development, the amount of a binder contained in the light-sensitive layer is preferably 1.5 to 10 g/m2, and more preferably 1.7 to 8 g/m2. The content of less than 1.5 g/m2 results in a marked density increase in the unexposed area, leading to levels unacceptable in practical use.
It is preferred to incorporate a matting agent to the image forming layer-side. Thus, it is preferred to allow a matting agent to exist on the surface of the photothermographic material to prevent images formed after thermal processing from abrasion. The amount of the matting agent is preferably 0.5 to 30% by weight, based on the whole binder of the light-sensitive layer-side. In cases where at least a non-image forming layer is provided on the side opposite to the light-sensitive layer, the non-image forming layer preferably contains a matting agent. The matting agent may be either regular form or irregular form, and preferably is a regular form and a spherical form is more preferred.
To control the amount or wavelength distribution of light passing through the light-sensitive layer, a filter dye layer may be provided on the light-sensitive layer side or an antihalation dye layer, a so-called backing layer may be provided on the opposite side. Alternatively, a dye or pigment may be incorporated into the light-sensitive layer.
Lubricants such as polysiloxane compounds, waxes or liquid paraffin may be incorporated in the light-insensitive layer, together with the foregoing binder or matting agent. 0068Various surfactants are used as a coating aid. Specifically, fluorinated surfactants are preferably used to improve antistatic properties or prevent dot-formed coating troubles.
The light-sensitive layer of the photothermographic material may be comprised of plural layers and to control the contrast, the light-sensitive layer may be arranged in the order of high-speed layer/low-speed layer or low-speed layer/high-speed layer.
Examples of suitable image toning agents used in the invention are described in Research Disclosure Item No. 17029 (June, 1978, page 9-15).
There may be incorporated mercapto compounds, disulfide compounds or thione compounds to control the thermal development speed by accelerating or retarding thermal development, to enhance spectral sensitization efficiency or to enhance storage stability before or after thermal development. There may also be used antifoggant, which may be incorporated into any one of the light-sensitive layer, light-insensitive layer or other layers. Furthermore, surfactants, antioxidants, stabilizers, plasticizers or covering aids may be used in photothermographic materials used in the invention. These additives and other additives described above are described in Research Disclosure Item No. 17029.
The support used in the invention is preferably a plastic resin film, such as polyethylene terephthalate, polycarbonate, polyimide, nylon, cellulose triacetate, and polyethylene naphthalate) to obtain an intended density or prevent image deformation after thermal development. of these, plastic resin support of polyethylene terephthalate or styrene type polymer having syndiotactic structure is more preferred. The support thickness is preferably 50 to 300 xcexcm, and more preferably 70 to 180 xcexcm. There may be used a plastic resin support which has been subjected to a thermal treatment. Plastic resins adopted therein are those described above. As a thermal treatment, the support is preferably heated at a temperature higher than the glass transition temperature of the support by at least 30xc2x0 C., more preferably at least 35xc2x0 C., and still more preferably at least 40xc2x0 C. Heating at a temperature exceeding the melting temperature of the support often vitiates uniformity in strength of the support.
Electrically conductive compounds, such as metal oxides and/or conductive polymers may be incorporated into the component layer to improve electrification properties. These compounds may be incorporated in any layer, and a sublayer, backing layer or interlayer between the light-sensitive layer and sublayer is preferred.
Photothermographic materials according to the invention are prepared using an organic solvent. One feature of the invention is that the photothermographic has a residual organic solvent content of 30 to 500 mg/m2.
Examples of organic solvents usable in the invention include ketones such as acetone, isophorone, ethyl amyl ketone, methyl ethyl ketone, methyl isobutyl ketone; alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, diacetone alcohol, cyclohexanol, and benzyl alcohol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol and hexylene glycol; ether alcohols such as ethylene glycol monomethyl ether, and diethylene glycol monomethyl ether; ethers such as ethyl ether, dioxane, and isopropyl ether; esters such as ethyl acetate, butyl acetate, amyl acetate, and isopropyl acetate; hydrocarbons such as n-pentane, n-hexane, n-heptane, cyclohexene, benzene, toluene, xylene; chlorinated compounds such as chloromethyl, chloromethylene, chloroform, and dichlorobenzene; amines such as monomethylamine, dimethylamine, triethanol amine, ethylenediamine, and triethylamine; and water, formaldehyde, dimethylformaldehyde, nitromethane, pyridine, toluidine, tetrahydrofuran and acetic acid. The solvents are not to be construed as limited to these examples. These solvents may be used alone or in combination. The solvent content in the photosensitive material can be adjusted by varying conditions such as temperature conditions in the drying stage after the coating stage. The solvent content can be determined by means of gas chromatography under conditions suitable for detecting the solvent and measured in the following manner. Thus, a photothermographic material is cut to a given size, which is to be accurately measured. This sample is finely chopped and sealed in a specified vial. After setting the vial onto a head space sampler, HP7694 (available from Hewlett-Packard Corp.) and heated to a prescribed temperature, the sample is introduced into gas chromatography. The solvent content can be determined by measuring the peak area of the intended solvent. All of the contained solvents cannot be determined by only one injection, so that measurement is made through the multi-space method by repeated injection of an identical sample. The residual organic solvent content is the total amount of the organic solvent remained in component layers including the light-sensitive layer side and backing layer side. The total residual organic solvent content of a photothermographic material used in the invention is 30 to 500 mg/m2, and preferably 100 to 300 mg/m2. The solvent content within the range described above leads to a thermally developable photosensitive material with low fog density as well as high sensitivity.
Image Formation Method
One feature of the image formation method of the invention concerns exposure of the photothermographic material using a light source having an emission region of relatively short wavelengths of 350 to 450 nm (preferably 370 to 420 nm), in place of near-infrared light of 600 to 800 nm, to form half tone dot images, thereby displaying effects of the invention (such as dot image density, linearity, contrast-increasing and prevention of residual dye staining). In addition thereto, incorporation of a decolorizing agent into the light-insensitive layer to absorb UV absorbent rays is preferred, thereby removing any remaining color due to the UV absorbent remaining in the thermally developed photothermographic material and leading to enhanced efficiency in the next step of printing onto a PS plate by Y UV rays.
In the image formation method of the invention, the photothermographic material is exposed to light to form half tone dot images using a short wave incoherent light source having an emission maximum at a wavelength of 350 to 450 nm (preferably 370 to 420 nm), thereby enhancing effects of the invention (i.e., achieving superiority in characteristics such as dot image density, linearity, contrast-increasing, residual dye stains). The reason thereof is not definitely clarified but it is supposed as follows. It is known that in conventional wet-process type photographic materials, the use of coherent light results in superior dot images compared to the use of incoherent light. However, it was confirmed by the experimental results according to the inventor of the present invention that in photothermographic materials relating to the invention, dot images with relatively high contrast and enhanced density were achieved by the use of incoherent light rather than the use of coherent light The reason for the difference in exposure between conventional wet-process type photographic materials and dry-process type photothermographic materials has not necessarily been clarified but it is contemplated to be attributed to the fact that the photothermographic material, silver halide grains are of a relatively small grain size, the light-sensitive layer being relatively thick and the organic silver salt existing in the form of needle-like crystals. Thus, it is supposed that such characteristics of the photothermographic material cause coherent light to be scattered, leading to deteriorated dot image quality.
The expression xe2x80x9cincoherentxe2x80x9d means the light phases being not the same, indicating that it is not a laser light but refers to an exposure light source for use in conventional silver salt photographic materials, so-called room light handling materials. Examples thereof include LED (Light Emission Diode), electrodeless lamp AEL (product by DAINIPPON SCREEN MFG. CO., LTD., daylight printer 647, emission wavelength of 360 to 440 nm), mercury lamp CHM-1000 (product by DAINIPPON SCREEN MFG. CO., LTD., daylight printer 607, emission wavelength of 360 to 440 nm), mercury lamp HL30201BF and UV lamp for use in exposure of PS plates.
To form dot images using an incoherent light source, the photothermographic material is subjected to dot-exposure to light which has been transmitted through a glass fiber from a light source.
Th e image formation method using incoherent light according to the invention leads to dot images with relatively high density and high contrast. In the future, it will be employed in a high-speed exposure apparatus, in which image signals are introduced into a liquid crystal panel provided on a light-transmittable support to form images and using the thus formed images as a master, t he photothermographic material is subjected to a single exposure (or exposure of one time) through a light source such as a UV lamp to form dot images.